According to the development of materials and development of devices which are carried out over a long period of time, the internal quantum efficiency of an organic electroluminescence (EL) element reaches almost 100%, and a further dramatic improvement of external quantum efficiency (EQE) depends only on how the outcoupling efficiency can be enhanced. As a technique for enhancing the outcoupling efficiency, a conventional method like using microlens or substrate with high refractive index is widely known (for example, Non Patent Literature 1), but it has a problem that cost required for the process or cost for substrate member is high. Furthermore, in recent years, a method of limiting light emission from transition dipole to the direction perpendicular to substrate by horizontal alignment of light emitting molecules is widely used (for example, Non Patent Literature 2). However, in terms of enhancement of the outcoupling efficiency, there is still large room for improvement, and thus a further improvement is in need.
Meanwhile, although experimental research for widely varying the refractive index of each semiconductor layer so as to control the light propagation within a device or the outcoupling efficiency has been widely carried out for a Group III-V inorganic semiconductor device like LED and laser diode, there are only limited examples of a study on organic semiconductor device (Patent Literature 1 and Non Patent Literature 3). There is a case in which a method of controlling the light propagation or outcoupling efficiency by adding an inorganic material layer on an outside of an organic semiconductor device is used. However, as the process cost is high and the refractive index inside a device cannot be directly controlled, there is also a problem that it is difficult to carry out the effective control of light propagation or outcoupling efficiency. There is also a problem that, when an inorganic material layer is formed on top of an organic semiconductor layer, it tends to damage the organic semiconductor layer as a lower layer.
Theoretical prediction stating that the outcoupling efficiency of an organic EL element can be enhanced by lowering the refractive index of an organic semiconductor thin film has been reported before (Patent Literature 1 and Non Patent Literatures 4 to 6). However, the biggest challenge for exhibiting the high effect is how to achieve a film with refractive index varying in broad range without impairing the electric property.
The amorphous organic semiconductor thin film used for an organic EL element or the like consists of π-conjugated organic materials with high molar refraction, and it is generally known to have refractive index of 1.7 to 1.8 or so in transparent region. However, because those of π-conjugated organic materials have a narrow range for controlling the refractive index, a significant enhancement of outcoupling efficiency cannot be expected from a constitution having simple organic semiconductor materials. Furthermore, according to the current state, the light propagation inside a device cannot be controlled by refractive index of a semiconductor itself like an inorganic semiconductor device, and thus it is necessary to use only an inorganic material layer (e.g., metal, conductive oxide, and insulating dielectric material) outside an element for controlling the light propagation. However, only with the control based on a member outside a device, the degree of freedom of optical design is low so that the light propagation inside a device cannot be sufficiently controlled. As such, development of an organic semiconductor material enabling control of refractive index is needed.